maybe the problem is in diseqc or in one of the cable!
To find out <try to connect one by one>:
-Disconnect all the lnbs from the diseqc.
-Connect one Lnb(ex:23.5) to diseqc port and remember that port.
-At the DM800 menu set 23.5 to that port (A=1,B=2 ... etc)
-If it was not working then that port of diseqc is damaged, try another port.
-now Connect another Lnb(ex:13.0) to unused diseqc port and check if u can get signals from both lnbs, and of course setting 13.0 to that port before testing.
and so on...
<assuming you know how to setup diseqc correctly at Receiver side>
I had similar problem and the problem was one of lnbs cable was short circuit!
try to strip both end of lnb cable and try again [ame="http://www.youtube.com/watch?v=hc1CTE9ee2Y"]How to connect satellite, LNB or coaxial cable to F Connector! - YouTube[/ame]